1. Field of the Invention
The present invention relates to a position detector using a semiconductor, a position detecting method using the same, and an optical rotary encoder, for detecting an incident position of a light spot which moves while describing an annular path.
2. Description of the Prior Art
FIG. 1 is a sectional view showing a conventional position detector. In the same figure, the reference numeral 1 denotes a photodiode which is in the shape of a flat plate, and numerals 2, 3 and 4 denote P, I and N layers, respectively, of the photodiode. Numerals 5a and 5b denote electrodes provided at both ends of the P layer 2 which is a surface layer, and numerals 6a and 6b denote output terminals extending from those electrodes. Numeral 7 denotes a bias electrode attached to the N layer 4 which constitutes the back of the photodiode. Numeral 8 denotes light which is incident on the photodiode 1, and numerals 9a and 9b denote photocurrents generated by the light 8.
The operation of such conventional position detector will now be described. Upon incidence of light 8 on the flat plate-like photodiode 1 in a certain position, an electric charge proportional to the energy of the light 8 is generated in the incident position. The electric charge thus generated passes through the P layer 2 as photocurrents 9a and 9b, which currents are outputted from the electrodes 5a and 5b, respectively. The electric charge in the incident position of the light 8 is divided into electric currents to both electrodes 5a and 5b in inverse proportion to resistance values of the P layer 2 portions up to the electrodes 5a and 5b, respectively. Since the P layer 2 is formed so as to be uniform in resistance value throughout the whole surface thereof, the division of current is inversely proportional to the distance from the incident position of the light 8 up to each of the electrodes 5a and 5b. Therefore, if the electric currents outputted from the electrodes 5a and 5b to the output terminals 6a and 6b are Ia and Ib, respectively, the spacing between the electrodes 5a and 5b is L, and the incident position of the light 8 with respect to the middle point between both electrodes which is assumed to be an origin, is X, the incident position of the light 8 can be detected in accordance with the following equation: EQU (Ia-Ib)/(Ia+Ib)=2X/L (1)
FIG. 2 shows the structure of another conventional position detector, which is described on page 125 of a collection of drafts for "The Third Robot Sensor Symposium" (1992). In the same figure, the numeral 11 denotes a flat plate-like photodiode which corresponds to the photodiode 1 in FIG. 1. Numeral 12 denotes a resistor serving as a dividing resistor, which is formed in parallel with the photodiode 11. Numeral 13 denotes an electrically conductive layer projecting onto the photodiode 11 at equal intervals like the teeth of a comb, and numeral 14 denotes an output electrode drawn out from the resistor 12, the output electrode 14 being provided in a plural number.
The operation of this position detector is basically the same as the position detector illustrated in FIG. 1, provided a difference is recognized in the following point. A photocurrent generated by the incidence of a light spot is conducted to the resistor 12 by means of the conductive layer 13 projecting onto the photodiode 11, whereby variations in the resistance value of the photodiode 11 are absorbed. A section area corresponding to the incidence of the light spot is selected on the basis of current values detected in the output electrodes 14 located at both ends and an incident position of the light spot is specified on the basis of current values in the output electrodes 14 disposed at both ends of the selected section area.
The following description is now provided about an optical rotary encoder. FIG. 3 is a diagram showing the construction of a conventional optical rotary encoder which is shown, for example, in "Optical Application Technique 1991, III-1" (Shadan Hojin: Optoelectro-Mechanics Association, published Apr. 30, 1991). In the same figure, the numeral 21 denotes a light emitting element, numeral 22 denotes a light receiving element disposed in an opposed relation to the light emitting element 21, and numeral 23 denotes a rotary slit disk fixed to a rotatable shaft in a position between the light emitting element 21 and the light receiving element 22. Numeral 24 denotes a fixed slit plate disposed between the light emitting element 21 and the light receiving element 22 together with the rotary slit disk 23. Numeral 25 denotes an amplifier for amplifying a signal obtained by the reception and conversion of light in the light receiving element 22 after passing through the rotary slit disk 23 and the fixed slit disk 24.
The operation of such conventional optical rotary encoder will now be described. In the rotary slit disk 23 are formed n number of concentric tracks constituted by slits corresponding to binary code bits which represent absolute angle addresses. Generally, the number of tracks, n, corresponds to the binary code length, n and it is determined by an angular resolution of the encoder used. In the illustrated example, n is set at 4. The fixed slit plate 24 is disposed on an optical path connecting the light emitting element 21 and the light receiving element 22. A rotational angle of the rotatable shaft is coded to the presence or absence of transmitted light from the rotary slit disk 23 and the fixed slit plate 24. This code is converted to an electric signal by the light receiving element 22, which signal is then amplified by the amplifier 25 and outputted.
According to the construction of the conventional position detector, as referred to above, the electrodes 5a and 5b are disposed at both ends of the flat plate-like photodiode 1. Therefore, in the case of forming the photodiode annularly for detecting an incident position of the light spot having an annular path, it is necessary to dispose two output electrodes 5a and 5b in any positions on the annular photodiode 1. In this construction, when the light spot is radiated to both electrodes 5a and 5b or to the gap between both electrodes, there arises a problem such that an output in the position detection is indefinite.
Since the conventional optical rotary encoder has the foregoing construction, in order to clearly detect the amount of light transmitted through openings of both rotary slit disk 23 and the fixed slit plate 24, it is necessary to dispose the disk 23 and the plate 24 in close proximity to each other. Particularly in the case of a high resolution encoder, the diffusion of light is marked because of a narrower slit width, so that a change in the spacing between the rotary slit disk 23 and the fixed slit plate 24 causes a great change in detection characteristics. Therefore, it is necessary to strictly control the spacing. There has also been a problem in that an eccentric condition caused by an error accidentally generated on fabricating the rotary slit disk 23 or an error accidentally generated on mounting the disk 23 to the rotatable shaft directly leads to an angle detection error.